The invention concerns a process for the manufacture of piezoceramic multilayer actuators.
A piezoceramic multilayer actuator 1 is shown schematically in FIG. 1. The actuator consists of stacked thin layers 2 of piezoelectrically active material, for example lead zirconate titanate (PZT), with conductive internal electrodes 3, which are led out alternately to the surface of the actuator, disposed between said layers. External electrodes 4, 5 interconnect the internal electrodes 3. As a result, the internal electrodes 3 are electrically connected in parallel and combined into two groups. The two external electrodes 4, 5 are the connecting poles of the actuator 1. They are connected via the connections 6 to a voltage source, not shown here. If an electrical voltage is applied via the connections 6 to the external electrodes 4, 5, this electrical voltage is transmitted in parallel to all internal electrodes 3 and produces an electric field in all layers 2 of the active material, which is consequently mechanically deformed. The sum of all of these mechanical deformations is available at the end faces of the head region 7 and the foot region 8 of the actuator 1 as a useable expansion 9 and/or force.
Piezoceramic multilayer actuators are fabricated according to the prior art as monoliths, that is to say the active material onto which internal electrodes are deposited by a silk screen process prior to sintering, is disposed as a so-called green film in successive layers as a stack that is compressed into a green body. The compression of the green body is usually carried out by lamination under the action of pressure and temperature in laminating moulds. Depending on the lamination tool used, this process determines to a large extent the external shape of the actuators. The laminate is separated into several actuators, which are pyrolized and then sintered. Since lamination in the unsintered body often produces non-homogeneity in the density, and the shrinkage during the firing of the ceramic is not of a constant value, the final required geometry of the actuators can only be accurately obtained by the hardening of the sintered actuators. However, in this case the internal electrode layers deposited in the actuators are thereby also processed. If the processed surfaces are not subsequently electrically insulated, then when these piezoceramic multilayer actuators are actively operated, there is a risk of an electrical flashover at the actuator surface because the dielectric field strength in air, which amounts to approximately 1000 V/mm, is exceeded by the operating field strengths of over 2000 V/mm. At the same time the smearing of the electrodes caused by the hardening additionally leads to reduced dielectric strength and/or leakage currents.
The object of the present invention is to present a process that simplifies the manufacture of multilayer actuators and by which the demonstrated disadvantages are avoided.
This object is achieved according to the present invention.
By stacking green films made of piezoceramic material, which are printed with the corresponding patterns of the internal electrodes for at least one piezoceramic multilayer actuator and by corresponding lamination under pressure of around 100 bar at a temperature of approximately 120° C., a green body is obtained with high mechanical strength, good adhesion between film layers, good mechanical machinability and homogeneous density. According to the invention, it is therefore possible to easily detach the multilayer actuators from such a green body as a laminate and subsequently by machining to yield their shape, which is usually already the final shape, so that after sintering the actuators require no further finishing. The insulating sinter skin needs only to be removed at the connecting faces where the internal electrodes have to be connected to the respective external electrode, for example by grinding. Due to the high mechanical strength of the laminate blocks, all machining operations, such as turning, milling, sawing, drilling or grinding are possible. In this case the bodies are neither damaged nor deformed. Due to the lower hardness of the material compared to the sintered state, tool wear is considerably reduced, thus making low-cost production possible.
Due to sintering, a so-called sinter skin forms all over the surface of the piezoceramic multilayer actuator, which sinter skin has such a high electrical insulating capability, even in the region where the internal electrodes emerge at the surface of the actuator, that subsequent insulation of the surfaces of the piezoceramic multilayer actuator can usually be omitted.
The sinter skin is thus abraded at the points where the internal electrodes are connected to the external electrodes.
Through suitable choice and/or combination of machining methods, the good machinability of the laminate enables piezoceramic multilayer actuators to be manufactured with different shapes. The cross-sectional areas can be circular, elliptical, square or polygonal. All edges of the green bodies can be broken, chamfered or rounded off prior to sintering. The ease of machining of the ceramic material in the green phase also enables rotationally symmetric mouldings to be produced.
The laminated block with the at least one multilayer actuator has a high strength and a high dimensional stability. It is thus possible, prior to sintering, to place several boreholes or pocket holes in the block and/or the unsintered piezoceramic multilayer actuator, which can additionally be provided with a thread. Such an arrangement can be advantageous for subsequent applications, such as fixings or connections. Since the layers of the internal electrodes are penetrated by the boreholes or pocket holes as well as by the machine-cut thread, in this case the sinter skin produced by sintering can also be advantageously used as an insulating layer.
A further option for shaping the multilayer actuators consists in punching holes of the required size, shape and number in the green films in the same operating cycle, prior to lamination, in which the green films are suitably punched out for the laminating mould. The green films with the printed internal electrodes thereon are then stacked one on top of the other in the required number and arrangement, so that the boreholes or pocket holes are produced in the desired arrangement and depth. Here again, threads can be machine-cut in the holes following lamination.
To increase stability and to maintain dimensional accuracy, the boreholes, through-holes or pocket holes can be filled prior to lamination with a filler which prevents any plastic deformation of the recesses which otherwise may occur during lamination. This filler is chosen so that under lamination conditions it is not more plastic or-cannot be deformed to a greater degree than the piezoceramic material of the green films.
A filler may consist of a hard, dimensionally stable and, during lamination, thermally stable material, for example metal or ceramic. According to the shaping, pins or threaded pins can be inserted into the boreholes or pocket holes.
Furthermore, plastic or thermoplastic fillers, in particular a highly-flexible rubber or-rubber-like plastic, are suitable. Here too the filler can have the form of a pin or threaded pin.
Following lamination, the pins or threaded pins are withdrawn or unscrewed from the laminate.
Fillers which remain dimensionally stable up to the lamination temperature are also suitable. During lamination or sintering these fillers smelt or pyrolize. For example, wax or low-melting-point polymers can be removed from the laminate by heating. A suitable organic material can also be used as a filler, such as is known from the prior art for forming porous ceramics, for example carbon black or a polymer that pyrolizes without residues during lamination or sintering at temperatures up to 700° C.
The thermal removal may also be achieved by melting out and/or thermal decomposition in a thermal process preceding sintering, for example an appropriate debonding process.
The invention is explained in further detail with the aid of an exemplifying embodiment in which: